In the last grant period this laboratory identified two of the cellular proteins bound by small and middle T antigens of polyoma virus as the 36kd catalytic and 63kd regulatory subunits of PP2A. We now propose to follow up our initial discovery by carrying out a careful genetic study of the structural requirements of the interaction. We will then proceed to use our mutants to attempt to gain insights into molecular details of the function of PP2A in vivo in both mammalian and insect systems. Finally we will attempt to push our understanding of the interaction to the atomic level via crystallography.